build: store SourceDateEpoch in manifest

The usage of granular `SOURCE_DATE_EPOCH` for packages is an
incrementing integer which could be useful for downstream tooling,
therefore add it to the packages manifest.

Signed-off-by: Paul Spooren <mail@aparcar.org>

include/package-ipkg.mk

@@ -181,6 +181,7 @@ $$(call addfield,Depends,$$(Package/$(1)/DEPENDS)
 )$$(call addfield,LicenseFiles,$(LICENSE_FILES)
 )$$(call addfield,Section,$(SECTION)
 )$$(call addfield,Require-User,$(USERID)
+)$$(call addfield,SourceDateEpoch,$(shell $(TOPDIR)/scripts/get_source_date_epoch.sh $(SOURCE))
 )$(if $(PKG_CPE_ID),CPE-ID: $(PKG_CPE_ID)
 )$(if $(filter hold,$(PKG_FLAGS)),Status: unknown hold not-installed
 )$(if $(filter essential,$(PKG_FLAGS)),Essential: yes

package/Makefile

@@ -84,7 +84,7 @@ $(curdir)/index: FORCE
 		mkdir -p $$d; \
 		cd $$d || continue; \
 		$(SCRIPT_DIR)/ipkg-make-index.sh . 2>&1 > Packages.manifest; \
-		grep -vE '^(Maintainer|LicenseFiles|Source|SourceName|Require)' Packages.manifest > Packages; \
+		grep -vE '^(Maintainer|LicenseFiles|Source|SourceName|Require|SourceDateEpoch)' Packages.manifest > Packages; \
 		case "$$(((64 + $$(stat -L -c%s Packages)) % 128))" in 110|111) \
 			$(call ERROR_MESSAGE,WARNING: Applying padding in $$d/Packages to workaround usign SHA-512 bug!); \
 			{ echo ""; echo ""; } >> Packages;; \